1. Field of the Invention
The present invention relates to an apparatus for detecting the interface between a hydrogen-containing substance and a second substance of different hydrogen content in a vessel or container. More particularly, the present invention relates to an apparatus for detecting the level of the interface of the hydrogen-containing substance in a vessel.
2. Description of the Background
A number of systems have been proposed which use radioactive sources for non-intrusive level detection, and control of liquid or solid levels in vessels. These devices generally use potent radioactive sources of gamma rays and suitable detectors to measure the attenuation of the gamma rays caused by level changes within the vessels. These devices suffer from many drawbacks, such as dangerously high levels of gamma radiation, expensive and heavy shielding requirements for safety, and non-linear response signals leading to poor accuracy and precision. These systems have been in commercial use for many years in spite of their many disadvantages.
A different type of level detector system uses an isotopic point source of fast neutrons in combination with a slow neutron detector tube. In this system, the portable source-detector assembly is usually mounted on a rod so that it can be easily moved up and down the walls of a vessel. For this technique to work, the material in the vessel must be or include a hydrogen-containing substance. Thus, the level of substance, such as water, hydrocarbons, aqueous solutions, organic molecules containing hydrogen and solids, such as grain or plastic powders, can be detected or determined. When the source-detector assembly is positioned on the vessel wall at a level below the level of the hydrogen-containing substance in the vessel, an abrupt increase in output signal is produced by the slow neutron detector tube. This is caused by fast neutrons from the source entering the vessel and interacting with the hydrogen atoms therein. The fast neutrons are slowed by these interactions (moderated) and deflected in different directions. Some are deflected through an angle of 180.degree. and are emitted from the vessel as slow neutrons which can now be detected by the slow neutron detector tube. The process by which these fast neutrons are slowed and reflected is generally referred to as "backscattering." If the source-detector assembly is now slowly raised, an abrupt drop in detector output signal occurs when the surface level (interface) is reached, since the number of backscattered neutrons drops sharply. The operator typically measures or marks the level of the liquid in the vessel by "searching" up and down to more accurately locate the level on the vessel where the signal change occurred. Accuracy of plus or minus one-half inch is generally obtained by this technique. The best accuracy is obtained from a very small "point" source of fast neutrons, and a horizontally oriented slow neutron detector tube. Both of these features tend to sharpen the "break" which indicates the liquid level in the vessel.
Another type of neutron backscattering system takes the form of a source-detector assembly mounted permanently on a vessel wall. If the liquid in the vessel rises above the source-detector assembly, an abrupt increase in signal due to backscattered neutrons is produced which can be used by conventional process control techniques to close valves to cause the level to drop and open again when the level falls below the control point. Such applications are referred to as "on-off" level controllers. This system also uses a point (small and very compact) source of fast neutrons mounted adjacent to a horizontally oriented slow neutron detector tube. This geometry is standard and was chosen to produce the most abrupt signal change possible in response to the changing interface level with respect to the detector to achieve improved accuracy of level detection and control. Ideally, the device would have a zero control span since it attempts to control at a single point. In practice, the control span is about plus or minus 0.5 inches.
Neutron backscattering detection has also been used in a portable source-detector assembly for locating air pockets in sunken ships or submarines. In this application, a diver "searches" over the hull of a sunken vessel with a source-detector assembly. The high backscatter signal from the surrounding water drops when the source detector reaches an air pocket inside the hull, indicating a good area to search for survivors.
In all of the above-described systems, the neutron detector tube is attached to an electronic console, which supplies high voltage and measures the number of voltage pulses caused by slow neutrons detected by the detector tube. This signal is usually referred to as the count rate. This count rate signal is readily displayed on an analog meter. It can be averaged, damped, amplified and used for process control as desired.
In all of the systems described above, a small, essentially point source of fast neutrons is employed. Moreover, the prior art systems basically only tell when a given interface is reached as opposed to having the ability to monitor, on a continuous basis, the location or change in level of the interface.